比如，下面这个表格，如何用stata计算每个月有几天Dretwd的值为0？
| Stkcd | Trddt | Opnprc | Hiprc | Loprc | Clsprc | Dnshrtrd | Dnvaltrd | Dsmvosd | Dsmvtll | Dretwd | Dretnd | Adjprcwd | Adjprcnd | Markettype | Capchgdt | Trdsta |
600033 | 2015-07-28 | 4.53 | 4.89 | 4.38 | 4.59 | 78948134 | 360721995 | 12596796 | 12596796 | -0.04375 | -0.04375 | 28.335923 | 18.756367 | 1 | 2014-06-09 | 1 |
600033 | 2015-07-29 | 4.66 | 4.9 | 4.53 | 4.89 | 56587591 | 266282916 | 13420116 | 13420116 | 0.065359 | 0.065359 | 30.187944 | 19.982273 | 1 | 2014-06-09 | 1 |
600033 | 2015-07-30 | 4.9 | 5.14 | 4.81 | 4.87 | 84491601 | 418089738 | 13365228 | 13365228 | -0.00409 | -0.00409 | 30.064476 | 19.900546 | 1 | 2014-06-09 | 1 |
600033 | 2015-07-31 | 4.76 | 4.89 | 4.61 | 4.72 | 44603125 | 212334652 | 12953568 | 12953568 | -0.030801 | -0.030801 | 29.138465 | 19.287593 | 1 | 2014-06-09 | 1 |
600033 | 2015-08-03 | 4.68 | 4.7 | 4.28 | 4.47 | 53806381 | 239984131 | 12267468 | 12267468 | -0.052966 | -0.052966 | 27.595114 | 18.266004 | 1 | 2014-06-09 | 1 |
600033 | 2015-08-04 | 4.49 | 4.75 | 4.48 | 4.75 | 50793499 | 235421851 | 13035900 | 13035900 | 0.06264 | 0.06264 | 29.323667 | 19.410183 | 1 | 2014-06-09 | 1 |
600033 | 2015-08-05 | 4.71 | 4.99 | 4.57 | 4.77 | 70792164 | 338452461 | 13090788 | 13090788 | 0.004211 | 0.004211 | 29.447135 | 19.49191 | 1 | 2014-06-09 | 1 |
600033 | 2015-08-06 | 4.62 | 4.74 | 4.57 | 4.63 | 35097583 | 163578842 | 12706572 | 12706572 | -0.02935 | -0.02935 | 28.582859 | 18.91982 | 1 | 2014-06-09 | 1 |
600033 | 2015-08-07 | 4.72 | 4.86 | 4.67 | 4.84 | 63590333 | 304760076 | 13282896 | 13282896 | 0.045356 | 0.045356 | 29.879274 | 19.777954 | 1 | 2014-06-09 | 1 |
600033 | 2015-08-10 | 4.83 | 5.11 | 4.83 | 5.06 | 78578644 | 393339401 | 13886664 | 13886664 | 0.045455 | 0.045455 | 31.237423 | 20.676952 | 1 | 2014-06-09 | 1 |
600033 | 2015-08-11 | 5.06 | 5.13 | 5.01 | 5.07 | 77584568 | 394166930 | 13914108 | 13914108 | 0.001976 | 0.001976 | 31.299157 | 20.717816 | 1 | 2014-06-09 | 1 |
600033 | 2015-08-12 | 5 | 5.19 | 4.95 | 4.99 | 99904709 | 506264464 | 13694556 | 13694556 | -0.015779 | -0.015779 | 30.805285 | 20.390908 | 1 | 2014-06-09 | 1 |
600033 | 2015-08-13 | 4.92 | 5.07 | 4.9 | 5.06 | 59375348 | 296492981 | 13886664 | 13886664 | 0.014028 | 0.014028 | 31.237423 | 20.676953 | 1 | 2014-06-09 | 1 |
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